add an SMP lock-stress test case

Four pairs push 400k sequenced messages through small channels; checks FIFO order and cross-core execution. Verified to fail with the lock disabled.

test/qemu_test.py

@@ -113,6 +113,12 @@ CASES = [
      "smp":    4,
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # Stress the big kernel lock across cores; heavier, so a longer timeout.
+    {"name": "smp-stress",
+     "smp":    4,
+     "timeout": 90,
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     {"name": "fault-ud", "expect": r"invalid opcode \(vector 6\)"},
     {"name": "fault-pf", "expect": r"page fault \(vector 14\)"},
     {"name": "fault-df", "expect": r"double fault \(vector 8\)"},
@@ -184,7 +190,8 @@ def run_case(arch, case):
         cmd += ["-smp", str(case["smp"])]
     qemu = subprocess.Popen(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
     try:
-        deadline = time.monotonic() + TIMEOUT
+        timeout = case.get("timeout", TIMEOUT)
+        deadline = time.monotonic() + timeout
         while time.monotonic() < deadline:
             time.sleep(0.2)
             text = ""
@@ -199,7 +206,7 @@ def run_case(arch, case):
                 if expect.search(text):
                     return True, "matched " + repr(case["expect"])
                 return False, "QEMU exited before matching (triple fault?)"
-        return False, f"timed out after {TIMEOUT}s without matching {case['expect']!r}"
+        return False, f"timed out after {timeout}s without matching {case['expect']!r}"
     finally:
         if qemu.poll() is None:
             qemu.terminate()